Pet Disguise
From HeRO Wiki
Located in Prontera behind a back alley, the "Veterinarian" (prontera 215, 260) can help you apply a "disguise" to your pet so that your pet looks like another pet that you have the pet egg for. You need to have a pet out along with other pet eggs, then the "Veterinarian" will change your current pet to look like one of your other pet eggs in your inventory. Pet disguises do not change your pet's attack power, stats, feed or abilities at all.
A disguised pet cannot equip or unequip its accessory. If you want to equip its accessory, you must remove the pet disguise.